Corona device



CORONA DEVICE Filed May 13, 1960 Win65 INVENTOR.

Roger Olden Attorney United States Patent 3,075,078. CORONA DEVICE Roger G. Olden, Princeton, N.J., assignor to Radio Corporation of America, a corporation of Delaware Filed May 13, 1960, Ser. No. 28,936 9 Claims. (Cl. 250-495) This invention relates generally to apparatus for charging insulating surfaces, and more particularly to improved apparatus for charging an electrophotographic surface to a substantially uniform negative potential. In electrophotography it is common to apply a uniform electrostatic charge to the surface of a photoconductive layer. The charge in selected areas is then dissipated by exposing the surface to a light image. The resulting pattern of charges is then rendered visible by applying thereto finely-divided developer particles which adhere to the surface by triboelectric attraction. Permanent visible images can be obtained, for example, by using thermoplastic developer particles which can be heat-fused to the photoconductive layer.

Some photoconductive layers require negative charging for optimum results. One such layer comprises particles of photoconductive white zinc oxide dispersed in an insulating binder. The zinc oxide is apparently an n-type semiconductor. Hence, the preference for negative charging. Zince oxide-binder layers are described in Electrofax Direct Electrophotographic Printing on Paper by C. J. Young and H. G. Greig, RCA Review, December 1954, vol. XV, No. 4. Charging is frequently and conveniently accomplished by exposing the surface of the photoconductive layer to a negative corona source. Such a source commonly consists of one or more fine wires in parallel array. When a voltage of the order of -5000 to l0,000 volts is applied to the wire or wires, a negative corona is generated and negative ions are attracted to and deposit on the surface of the photoconductive layer to produce the negative charge thereon.

Production of negative corona with a fine wire or wires presents problems not encountered in the production of positive corona. The generation of positive corona appears as a continuous uniform visible sheath surrounding the Wire, whereas thenegative corona has a tendency to concentrate at discrete points along the wire appearing and disappearing at different sites. Ions, from the negative corona, deposit on a photoconductive layer in nonuniform pattern resulting in non-uniform electrophotographic printing. Attempts to eliminate such bright spots in the corona discharge, by providing a highly polished continuous surface on the corona wire, have not been successful.

Non-uniformity in the charge deposited on the photoconductive surface resulting from negative corona generation can be overcome by providing two dimensional relative movement between the charger and the photoconductive layer, and/or by making multiple passes of the charger with respect to the layer. In this way, the uneveness of charging, attributable to a fixed corona wire, is averaged out over the surface of the plate as it is charged. Charging in this manner can only be accomplished in mechanized operations by relatively complicated and expensive apparatus.

It is a general object of this invention to provide improved and simplified corona generating apparatus.

It is a further object of this invention to provide improved electrostatic charging apparatus wherein negative corona can be generated with a substantially improved uniformity.

It is yet another object to provide improved charging apparatus including at least one corona generating wire capable of producing substantially uniform negative charge on an insulating surface.

Generally the foregoing and other objects and advantages are achieved in accordance with the invention by incorporating in a corona charging apparatus at least one corona generating wire having a discontinuous surface which provides ,a closely-spaced multiplicity of discrete corona generating points therealong. As defined herein, a discontinuous surface is intended to include one having a multiplicity of pits therein, one which has been me chanically or otherwise distorted to disrupt the smoothness thereof, or one which has been provided with a multiplicity of undulations. Such a wire may comprise a single strand the surface of which has been sand-blasted or etched to provide a multiplicity of discontinuities therealong. Alternatively, or in addition, the 'wire or wires comprise two or more strands which have been twisted together to form a wire rope with an undulated surface. It has been discovered that improved uniformity of negative charging is achieved by practicing this invention.

Further details are provided in the following description andare illustrated in the drawings wherein:

FIG. 1 is a plan view of a single wire corona source constructed in accordance with this invention.

FIG. 2 is a plan view of another embodiment of this invention the top plate thereof being cut away to show a plurality of corona wires within the structure.

"Similar reference characters are applied to similar elements throughout the drawings.

In the embodiment of FIG. 1, a corona generating wire 11 is supported in a frame made of of two side rails 13 and 15 and two end bars 17 and 19 fastened together to form an elongated rectangle. The side rails 13 and 15 may comprise insulating or conductive material while the two end bars 17 and 19 preferably are made of insulating material. The corona generating wire is supported within the frame by the two end bars 17 and 19. Preferably the corona generating wire is attached to one end bar 17 by means of a spring 21 and to the other end bar 19 by a fixed fastener 23. In this way a constant tension may be maintained on the corona generating wire 11 While the fixed fastener 23 provides connection means for applying ahigh negative potential to the corona generating wire 11.

The corona generating wire 11 may be provided with a discontinuous surface in a variety of ways. For example, a smooth surfaced single strand may be acid etched or sand-blasted to provide a multiplicity of peaks and valleys along the surface of the wire. An undulated surface can be achieved by spirally wrapping one wire around another. A preferred corona generating wire 11 is simply constructed by twisting together two or more strands to form a wire rope having an undulated surface.

A specific example of a corona generating wire 11 comprises a wire rope of two strands each having a diameter of 0.002 inch. The strands are twisted together with a pitch of from about A.; to about 3 inch. When about 5 to 10 kilovolts negative are applied to such a wire a negative corona is generated which closely resembles the sheath-like appearance of a positive corona. With such a wire, a wide band of uniform charge is applied to an insulating layer when the corona wire is held stationary and the entire surface of an insulating layer may be uniformly charged by passing the corona wire in a single pass thereover in one direction only, i.e. transverse to the length of the wire.

In FIG. 2, there is illustrated corona generating apparatus having a plurality of corona generating wires 11 substantially equidistant apart and parallel to one another. Each corona wire is mounted within a rectangular frame in the same manner as depicted in FIG. 1. In the appara tus of FIG. 2, there is also included a flat conductive shield 25 parallel to and on one side of the corona wires 11. Such a shield facilitates corona generation and uniform charge deposition.

acvepvs c a a In a typical apparatus, constructed as shown in FIG. 2, the corona wires 11 are spaced apart about :4 inch up to about 1 /2 inches, /8 inch from the surface to be charged, and inch from the conductiveshield 25. The dimensions set forth herein are not critical, being provided to illustrate only one typical structure in accordance with this invention. For example, corona wiresof larger diameter may be'employed but will ordinarily require higher applied voltages for corona generation. Smaller diameter wires may be employed, however, such wires will be more difiicult to fabricate. i The apparatus of either FIG. 1 or FIG. 2 may be employed for charging photoconductive members in electrophotographic processes such as, for'exarnple, the reproductionof images on paper having a photoconductive coating on one side thereof. The paper is placed on a grounded conductive backing plate, coated side up. The coating is then uniformly charged with the corona generating apparatus of FIG. 1 or 2, with from about 5000 to 10,000

volts negative corona generating potential applied to the or the apparatus may be moved transversely to the length of the wire or wires to cause charge to impinge on all areas of the coating. In continuous printing operations continuous paper webs are frequently employed. In such operations, improved charging can be Obtained by employing a second positive corona generating apparatus in place of the aforesaid conductive backing plate as described in US. Patent 2,922,883 to E. C. Giaimo, Jr.

What is claimed is: a

l. Corona discharge apparatus for applying electrostatic charge on asurface of an electrostatic recording element, said apparatus comprising a support member and at least one fine conductive wire carried by said support member, said wire having a'rnultiplicity ofcorona generating sites therealong consisting of a multiplicity of closely spaced discontinuities in the surface thereof, and connection means associated with said Wire for applying other strand coiled thereabout.

4. The corona discharge apparatus of cla'un l including a conductive shield spaced from said wireand substantially parallel thereto.

5. The apparatus of claim 4 wherein said wire comprises a plurality of fine strands twisted together.

6. Corona discharge apparatus for applying electrostatic charge on a surface of an electrostatic recording element, said apparatuscomprising an insulating support member carrying a plurality of fine conductive wires in spaced apart parallel relation with respect to one another and with. respect to said surface, each said wire having a multiplicity of corona generating sites therealong consisting of a multiplicity of closely spaced discontinuities in the surface thereof, and connection means associated with said wires for applying thereto corona generating potential to produce said charge on said surface.

7. The apparatus of claim 6 wherein each said wire comprises a plurality of fine strands twisted together.

8. Corona discharge apparatus for applying electrostatic charge on a surface of an electrostatic recording element, said apparatus comprising an insulating support carrying a plurality of fine wires in spaced apart parallel relation with respect to one another and with respect to said surface, each said wire having a multiplicity of corona generating sites therealong consisting of a multiplicity of closely spaced discontinuities in the surface thereof, connection means associated with said wires for applying thereto corona generating potential to produce said charge on said surface, and a conductive shield on one side of said wires and substantially parallel thereto.

9. The apparatus of claim 8 wherein each said wire comprises a plurality of strands twisted together.

References Cited in the tile of this patent UNITED STATES PATENTS 1,515,293 Whitehead Nov. 11, 1924 1,56l,249 Kraut Nov. 10, 1925 1,989,670 Anthony Feb. 5, 1935 2,590,323 .lanser et al. Mar. 25, 1952 2,922,883 Giaimo Jan. 26, 1960 

1. CORONA DISCHARGE APPARATUS FOR APPLYING ELECTROSTATIC CHARGE ON A SURFACE OF AN ELECTROSTATIC RECORDING ELEMENT, SAID APPARATUS COMPRISING A SUPPORT MEMBER AND AT LEAST ONE FINE CONDUCTIVE WIRE CARRIED BY SAID SUPPORT MEMBER, SAID WIRE HAVING A MULTIPLICITY OF CORONA GENERATING SITES THEREALONG CONSISTING OF A MULTIPLICITY OF CLOSELY SPACED DISCONTINUITIES IN THE SURFACE THEREOF, AND CONNECTION MEANS ASSOCIATED WITH SAID WIRE FOR APPLYING THERETO CORONA GENERATING POTENTIAL TO PRODUCE SAID CHARGE ON SAID SURFACE. 